Conventional electrostatic copiers rely heavily on the ability of a corona generator to charge a photoconductor to a selected potential, and to do so uniformly across the photoconductor. Failure to charge the photoconductor to the desired potential reduces the contrast available in the print. Lack of uniformity in charging across the photoconductor produces degraded prints which have "streaks" or varying contrast.
One form of the conventional corona generator, the so-called scorotron, includes a housing with a plurality of conductive wires located between the housing and the photoconductive layer of the copier. For control purposes, a shield, comprising a further plurality of wires is located between the first group of wires and the photoconductor. The wires and the shield have applied to them selected potentials, see, for example, U.S. Pat. No. 2,777,957. One technique used in the prior art to monitor the operability of a corona generator is to measure the current delivered to the generator by the power supply. The presence of the shield, however, complicates the measurement problem for at least some of the current to the corona generator is drawn off by the shield and does not result in charging the photoconductor.
Another technique is to actually remove the normal photoconductor (usually requiring removal of the photoconductor carrier, i.e., a drum) and substitute a special drum with insulated areas for measurement purposes. The disadvantage of this technique, aside from the drum removal requirement, is the error produced when the normal drum and special drum differ in size or shape. In addition, the substitute drum is typically not rotated and so measurements are made with the machine in a static condition and thus the measurements are poor analogs of actual machine operation which is dynamic.
The most effective technique to monitor the performance of the corona generator is to measure the current flowing between a conductive base layer of the photoconductor and ground. This has been effected by insulating the conductive base layer from ground and employing a current measurement device between the insulated conductive layer and ground; in this regard, see U.S. Pat. Nos. 3,950,680; 3,335,274; and 3,335,275.
As mentioned in U.S. Pat. No. 3,950,680, a drawback of this technique is especially evident in multiple corona machines in that conductive layer current is typically not due to a single corona generator. Rather, conventionally, the multiple corona machines have more than one corona energized at a time, and therefore, the current flowing in the conductive layer is the result of more than one corona generator. As a result, the current measurement does not provide an accurate measure of operation of any single one of the corona generators.
Furthermore, measurement of conductive layer current is incapable of indicating corona alignment, even if only a single corona generator is energized.
It is therefore one object of the present invention to provide apparatus for accurate measurement of the proper operation of a corona generator in a multiple corona generator electrostatic copier. It is another object of the invention to provide apparatus of the foregoing type which is capable of monitoring operation of a corona generator without requiring removal of the normal photoconductor and substitution of apparatus therefor. It is a further object of the present invention to provide apparatus of the foregoing type in which a normal ground path for the conductive layer is interrupted for measurement purposes but which provides a signal to the electrostatic copier control unit, indicating whether or not the normal ground path is made.
It is another object of the present invention to provide a method of corona alignment for use with the inventive apparatus.
It is yet another object of the present invention to provide the method of corona alignment which provides for rapid, safe and trouble-free measurement of corona alignment. Another object of the present invention is to provide a dynamic method of measuring corona drum current where the photoconductor potential is similar to normal operating conditions and does not saturate as it would if the drum were not rotating.